Automatic brake adjuster

ABSTRACT

An automatic brake adjuster for compensating for the wear of the brake friction surfaces. The brake piston has a centrally located threaded hole which is resiliently engaged by a threaded split stem. The stem is spring biased toward the closed end of the brake cylinder but upon application of fluid pressure to the cylinder, the piston and the stem move together for a limited distance toward the open end of the cylinder. The stem contacts a stop which prevents it from moving any farther toward the open end of the cylinder while the continued pressure against the piston urges it toward the open end of the cylinder with sufficient force to cause the split portion of the stem to deflect radially inwardly and permit the threaded portion of the cylinder to skip across the threaded portion of the stem until the piston has traveled outwardly a sufficient distance to move the brake friction surfaces in contact and apply the brake. When the brake fluid pressure is released from the cylinder, the spring causes the stem to return to its original retracted position against the closed end of the cylinder. When sufficient wear has occurred in the brake to require adjustment of the clearance, the next time the brake is applied, the piston will skip one or more threads on the stem and position itself axially outwardly on the stem so that when the brake pressure is released and the stem returns to its retracted position, the outer end of the piston will remain in a sufficiently extended position to compensate for the change in brake clearance due to wear.

UnitedStates Patent [72] Inventor Richard Crossman Tallrnadge, Ohio [21]Appl. No. 767,324 [22] Filed Oct. 14, 1968 [45] Patented Jan. 12, 1971 v[73] Assignee The Goodyear Tire & Rubber Company Akron, Ohio acorporation of Ohio [54] AUTOMATIC BRAKE ADJUSTER 11 Claims, 6DrawingFigs.

[52] US. Cl 188/196, 7 l88/71.8 [51] Int. Cl Fl6d 65/54 [50] FieldofSearch 188/72-73C, 79.5GE, 196P,R, 71.8

[56] 1 References Cited UNITED STATES PATENTS 2,568,858 9/1951 Kovac ..l88/196(R)UX 2,736,396 2/1956 Rasmussen et al.. 188/196(R) 3,338,3548/1967 Jeffries 188/73(C)X Primary Examiner-Duane A. Reger Att0rneysF.W. Brunner and Paul E. Milliken moving any farther toward the open endof the cylinder while the continued pressure against the piston urges ittoward the open end of the cylinder with sufficient force to cause thesplit portion of the stem to deflect radially inwardly and permit thethreaded portion of the cylinder to skip across the threaded portion ofthe stem until the piston has traveled outwardly a sufficient distanceto move the brake friction surfaces in contact and apply the brake. Whenthe brake fluid pressure is released from the cylinder, the springcauses the stem to return to its original retracted position against theclosed end of the cylinder. When sufficient wear has occurred in thebrake to require adjustment of the clearance, the next time the brake isapplied, the piston will skip one or more threads on the stem andposition itself axially outwardly on the stern so that when the brakepressure is released and the stem returns to its retracted position, theouter end of the piston will remain in a sufficiently extended positionto compensate for the change in brake clearance due to wear.

PATENTEDJAN12|97| 3.654.336

SHEEI10E3 INVENTOR. RICHARD CROSSMAN ATTORNEY PATENTEDJA'mmn 3,554,33

' sum 3 or 3 PEG. 6

48 INVENTOR.

RICHARD L. CROSSMAN f/ /;4/

ATTORNEY AUTOMATIC BRAKE ADJUSTER This invention relates to an automaticbrake adjuster for adjusting the travel of a brake'piston to compensatefor wear of the brake friction surfaces.

BACKGROUND OF THE INVENTION In the prior art, there are many ways ofadjusting brake travel to compensate for wear of the brake frictionsurfaces. Many-such adjustment devices require manual adjustmentperiodically andthose adjusters which operate automatically are oftencumbersome and complicated and have many moving parts. The closest priorart known to the applicant is U.S. Pat. No. 3,365,029 issued to H. C.Swift. The Swift patent, however, is a more complicated device than theApplicants and, in addition to being used with a hydraulic brake, isalso designed to accommodate a manually operated brake such as 'aparking brake.

OBJECTS or THE INVENTION It is theprimary object of this invention toprovide an automatic brake adjuster which is simple in construction,lighter in weight, less expensive, and contains a minimum number ofmoving parts.

Another object of this invention is to provide an automatic brakeadjuster which is simple to assemble and requires a minimum amountofmaintenance.

' BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a cross-sectional viewshowing oneembodiment of the inventionwith the brake piston in aretracted position;

, FIG. 2 is a cross-sectional view of the same embodiment shown in FIG.1 but with the piston in an extended position and with an adjustmenthaving been'made by the adjustment mechanism;

FIG. 3 is a cross-sectional view of the embodiment shown in FIG. I butwith the piston in a retracted position after adjustment has been made;1

FIG. 4 is an exploded perspective view showing the con struction of thesplit stem of the invention and a retaining nut which is threaded ontothe solid end of the stem;

FIG. 5 is a fragmentary cross-sectional view showing another embodimentof the invention; and

FIG. 6 is a partially broken away side view of another type of pistonwhich may be used with the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring now to FIG. 1, abrake housing and piston assembly is indicated generally by thenumeral 1. The assembly I is normally fastened to a torque plate orother fixed member on a vehicle and is positioned adjacent one or morebrake discs or friction surfaces. The present invention may be used inconjunction with a brake having a-plurality of rotary discs interleavedbetween a plurality of stationary discs such as may be seen in U.S. Pat.No. 3,2l7,844 issued to J. P. Nelson et al. It will also be understoodthat this invention may be also used with other types of brakes whichrequire a fluid pressure operated piston for actuating the brakefriction members. Since the invention resides primarily in theadjustment mechanism, the rest of the brake, including the brake discs,the axle and the wheel, will not be shown in the interest of simplicity.The assembly 1 contains a housing 2 having a cylinder bore 3. A fluidpressure inlet 4 is connected to a brake fluid pressure line (not shown)for bringing hydraulic fluid into the cylinder bore 3. A cylinder sleeve5 is threaded into the cylinder 3 by a threaded portion 6 on the outsidecircumference of the sleeve 5 which engages an internal threaded portion7 inside the cylinder 3. The sleeve 5 has a radially outwardly extendingflange 8 which seats against the housing 2 and serves as a stop todetermine how far the sleeve 5 may be threaded into the cylinder 3. AnO-ring 9 is positioned between the housing 2 and the flange 8 to providea leakproof seal to prevent fluid pressure from leaking between thesleeve 5 and the housing 2. The sleeve 5 retains a piston 10 and theentire adjustment mechanism 11 within the cylinder 3 as will bedescribed in further detail hereafter. The piston 10 is slidably carriedwithin the sleeve 5. An annular groove 12 located on the radially outersurface of the piston 10 at the axially inner end thereof carries anO-ring 13 which seals against the radially inner surface of the sleeve 5to prevent escape of hydraulic fluid from the cylinder 3. The piston 10also has a hollow bore 14 at the axially inner end for containing aportion of the adjustment mechanism 11. A threaded center bore 15extends into the portion of the cylinder 10 lying axially outwardly fromthe bore 14. A circular insulator I6 is attached to the axially outerend of the piston 10 by a pair of screws 17. A typical elastomerie dustseal ring 18 is connected to annular groove 19 in the sleeve 5 and isgripped between the piston 10 and the insulator 16. The dust seal ringwill permit the piston 10 to slide freely in and out of the cylinder 3but will prevent particles of dust and dirt from entering between thepiston 10 and the sleeve 5 and thereby causing wear of the piston andsleeve or from inhibiting the operation of the piston. The insulator 16is attached to the piston to prevent heat transfer from the pressureplate 20 and from the brake discs (not shown) against which the pressureplate 20 is brought to bear by actuation of the piston 10. When thebrakes are applied, the heat generated by the friction of the discsrubbing together will be transferred through the pressure plate andwithout an insulator such as the insulator 16 will transfer to thepiston and will cause damage to the piston or other brake parts or willcreate other problems which will prevent efficient operation of thebrake. It will be understood that in some types of brakes inv volvingthe use of nonmetallic friction pads in combination with brake discsrather than a combination of stationary and rotary metallic discs, afriction pad maybe attached directly to the outer end of the piston byany conventional connector means, in the same manner as the insulator16.

In the'embodiment illustrated by FIG. I, it is contemplated that thepressure plate 20 will bear against an adjacent set of metal discs suchas those used in abrake of the type shown and described in US. Pat. No.3,217,844 as previously mentioned.

The adjustment mechanism 11 has a. hat-shaped retaining cup 21 which isheld in position within the cylinder 3 by the axially inner end of thesleeve 5 which contacts a ledge 22 near the outer circumference of aflange 21a of the cup 21. A threaded stem 23, as shown in FIG. 4, has asplit threaded portion 24 having a pair of splits 25 and 26 passingthrough the center axis of the stem 23, with the splits 25 and 26 beinglocated at to each other. The splits in the threaded portion 24 permitthis part of the stem to be deflected radially inwardly to reduce thestem diameter as will be described later in the specification. Inaddition to the split portion 24, the stem 23 has a solid portion 27 ofsmaller diameter than the portion 24. The portion 27 has threads 28 forreceiving the stem nut 29 as shown assembled in position in FIG. I. Thestem nut 29 has a plurality of holes 30 to permit the passage of fluidpressure therethrough. When assembled in position, the stem 23 has thesplit threaded portion 24 screwed into the threaded bore 15 of thepiston 10. The solid portion 27 of the stem extends through centrallylocated hole 31 in the retaining cup 21. A spring 32 is placed in theretaining cup 21 surrounding the solid portion 27 of the stem 23, andthe stem nut 29 is screwed onto the threaded portion 28 at the axiallyinner end of the stem 23 to bear against the spring 32 and hold it inposition. The piston and the entire adjustment mechanism 11 are held inposition in the cylinder 3 by the sleeve 5. It may be seen that a smallclearance 33 is provided between the retaining cup 21 and the back wallof the cylinder 3 to permit the flow of hydraulic fluid to act upon thepiston 10. With the piston 10 in the adjustment mechanism 11 assembledas shown in FIG. 1, it may be seen that the spring 32 will bear againstthe retaining cup 21 and the stem nut 29 and urge them apart. Since thesplit portion 24 is of larger diameter than the hole 31 in the retainingcup 21, the spring 32 will urge the nut 29 and the stem 23 toward theleft until the end 34 of the split portion 24 comes in contact with thecup 21. The end 34 then serves as a stop to prevent further movement ofthe stem 23 with respect to the retaining cup 21. When fluid pressure isintroduced through the inlet 4 into the cylinder 3, it passes throughthe clearance area 33 through the holes 30 in the nut 29 and through theholes 35 and 36 in the retaining cup 21. All these holes permit thefluid pressure to pass freely through the parts of the adjustmentassembly 11 and bear against the inner end of the piston 10. When thehydraulic pressure in the cylinder 3 is sufficient to overcome theresistance of the spring 32, the piston 10 moves to the right as shownin FIG. 2 and brings with it the stem 23 and the stem nut 29. The stemnut 29 compresses the spring 32 and moves to the right until it reachesa stop ledge 37 on the radially inner face of theretaining cup 21. Theledge 37 prevents further movement to the right of either the stem 23 orthe stem nut 29. If there is still clearance in the brake when thepiston has moved this far, the fluid pressure in the cylinder 3 willcause the piston to continue to the right a sufficient additionaldistance to apply the brakes. The splits 25 and 26 in the split threadedportion 24 provide sufficient resiliency to permit the threaded portion24 to deflect radially inwardly and thereby decrease its diameter asufficient amount to permit the threads of the threaded bore in thepiston to skip across the threads of the split threaded portion 24. Thispermits the piston 10 to move axially with respect to the stem 23 whilethe stem is being held in a fixed position by the retaining cup 21. Whenthe brakes are applied and there is excess clearance between the pistonand the friction elements of the brake, the piston 10 will skip one ormore threads on the stem 23 depending upon the amount of adjustmentrequired. Upon release of pressure in the cylinder 3, the spring 32 willmove the stem 23 and the piston 10 to the retracted position shown inFIG. 3. It may be seen in FIG. 3 that although the stem 23 and the stemnut 29 have returned to the same position they were in FIG. 1, thepiston 10 does not return to the same position within the cylinder butremains slightly extended due to the additional movement of the pistonby skipping threads on the stem when the brake is applied, as shown inFIG. 2.

As wear continues on the brake friction elements, the piston willcontinue to reposition itself further and further axially outwardlyalong the stem 23 until the friction elements are sufficiently worn thatthey must be replaced. Upon replacement of the friction elements, thepiston 10 is again repositioned to the axially inward position shown inFIG. 1, and the piston again gradually works it way axially outwardly asthe friction members continue to wear.

In order to prevent rotation of the piston 10 with respect to the stem23, it has been found necessary to provide a U- shaped clip 38, one partof which is inserted in a hole 39 in the piston 10 and the opposite partprojects loosely into the split 26 in the stem 23. The clip 38 permitsthe stem 23 and the piston 10 to be moved freely axially in eitherdirection with respect to each other but will prevent relativecircumferential movement between the stem and the piston. The clip 38 isnecessary since unwanted circumferential movement would result inundesired axial movement of the piston 10 on the stem 23 which wouldresult in improper adjustment of the brake clearance. Although in FIGS.1 through 3, the piston and adjustment mechanism is shown held inposition within the brake cylinder by a cylinder sleeve 5 in someinstances, it is desirable to eliminate the use of the cylinder sleeve.

In FIG. 5, the adjustment assembly 11 and the piston I0 are held inposition by a lock ring 40 which bears against the retaining cup 21 andengages an annular groove 41 in the housing 2. In FIG. 5, a seal ring 42is located in an annular groove 43 in the inner surface of the cylinder3 at the axially outer end thereof rather than the seal being located onthe axially inner end of the piston as shown in FIGS. 1 through 3. Anannular groove 44 is provided at the axially outer end of the cylinder 3to receive a dust seal ring 45 similar to the ring 18 in the embodimentshown in FIG. 1. In some instances, it may be desirable to make thepiston 10 and the insulator 16 as one integral part as shown in FIG. 6in which an insulator piston is represented by the numeral 46. Thepiston 46 may be made of any suitable insulating material which is ofsufficient strength to withstand the pressures and stresses to which itmay be subjected. The piston 46 has an annular groove 47 for receivingan O-ring seal similar to the seal 13 shown in FIG. 1. A U- shaped clip38 similar to that shown in FIG. 1 is also used with the piston 46. Whenthe piston is made of a nonmetallic material such as the piston 46 inFIG. 6, or when the piston is made of a soft metal such as aluminum, thethreaded bore of the piston would not be sufficiently strong towithstand the relative axial movement between the stem and the pistonwithout stripping of the threads within the bore of the piston. Toremedy this situation when softer materials are used for the piston, ametallic threaded insert 48 of a hard metal such as steel is inserted inthe threaded center bore 49 of the piston 46. Such insert presents ahard threaded portion for engaging the threaded portion of the stem. Thethreads of the insert 48 are sufficiently hard that they will not bestripped by continued adjustment of the brake. It will be understoodthat it is also necessary for the threads on the threaded portion of thestem23 to be of some hard material such as steel or the like.

While the piston and the adjustment assembly have been shown in thedrawings as held in place within the cylinder. either by a cylindersleeve or by a retaining ring, it will also be realized that it ispossible to attach the retaining cup 21 to the cylinder by screws orother well known fastening devices without departing from the scope ofthe invention. It should also be emphasized that it is immaterialwhether the seal ring creating a seal between the piston and thecylinder is located on the inner surface of the cylinder or on the outersurface of the piston. A brake friction pad may be substituted for theinsulator shown in FIGS. 1 through 3 without departing from the scope ofthe invention.

Various other changes can be made in the illustrative embodiments shownherein without departing from the scope of the invention.

I claim:

1. An automatic brake adjuster for adjusting the distance of travel of abrake piston in a cylinder to compensate for wear of the brake frictionsurfaces comprising:

A. a housing having a cylinder;

B. a piston within the cylinder, said piston having a threaded portion;

C. a thread-engaging means for resiliently contacting at least part ofthe threaded portion of the piston to permit relative axial movement ofthe cylinder and the threadengaging means with respect to each otherwithout relative circumferential rotation of either of said parts;

D. retaining means mounted within the cylinder for permitting limitedaxial, movement of the thread-engaging means with the piston uponapplication of the brake; and

E. a sleeve threadably attached to the inner wall of the cylinder forholding the retaining means.

2. An automatic brake adjuster as claimed in claim 1 wherein thethreaded portion of the piston is in a hole located centrally in thepiston and the thread-engaging means is a threaded stem having at leastone split through the portion thereofwhich engages the threaded hole inthe piston, the split permitting the stem to decrease in diameter asufficient amount to permit the threads of the stem to skipacross thethreads of the piston when the piston and stem are moved axially withrespect to each other.

3. An automatic brake adjuster as claimed in claim 1 wherein theretaining means comprises a retaining cup mounted in a substantiallyfixed axial position within the cylinder and'a spring within theretaining cup biasing the limited distance with the piston toward anextended position.

4. An automatic brake adjuster as claimed in claim 1 wherein the pistonis made of metal and has a nonmetallic insulator attached to the axiallyouter end thereof.

5. An automatic brake adjuster as claimed in claim 1 wherein the pistonbody is made from a nonmetallic insulating material and has a metalthreaded insert located centrally thereof.

6. An automatic brake adjuster for adjusting the distance of travel of abrake piston in a cylinder to compensate for wear of the brake frictionsurfaces comprising:

A. a housing having a cylinder;

B. a piston within the cylinder, said piston having a threaded portion;

C. a thread-engaging means for resiliently contacting at least part ofthe threaded portion of the piston to permit relative axial movement ofthe cylinder and the threadengaging means with respect to each otherwithout relative circumferential rotation of either of said parts;

i D. means mounted within the cylinder for permitting limited axialmovement of the thread-engaging means with the piston upon applicationof the brake; and

E. a lock ring engaging an annular groove in the axially inner end ofthe cylinder for holding the retaining means.

7. An automatic brake adjuster as claimed in claim 6 I wherein thethreaded portion of the piston is in a hole located centrally in thepiston and the thread-engaging means is a threaded stem having at leastone split through the portion thereof which engages the threaded hole inthe piston, the split permitting the stem to decrease in diameter asufficient amount to permit the threads of the stem to skip across thethreads of the piston when the piston and stem are moved axially withrespect to each other.

8. An automatic brake adjuster as claimed in claim 6 wherein theretaining means comprises a retaining cup mounted in a substantiallyfixed axial position within the cylinder and a spring within theretaining cup biasing the threadengaging means in a retracted positionwithin the cylinder but upon sufficient pressure against the pistonwithin the cylinder pennitting the thread-engaging means to travel alimited distance with the piston toward an extended position.

9. In a disc brake having a pressure plate for exerting pressure againstthe brake discs, an automatic brake adjuster for adjusting the distanceof travel of a brake piston in a cylinder to compensate for wear of thebrake friction surfaces compristng:

A. a housing having a cylinder;

B. a piston within the cylinder, said piston having a threaded portion;

C. a thread-engaging means for resiliently contacting at least part ofthe threaded portion of the piston to permit relative axial movement ofthe cylinder and the threadengaging means with respect to each otherwithout relative circumferential rotation of either of said parts;

D. a retaining cup mounted in a substantially fixed axial positionwithin the cylinder; and

E. a spring within the retaining cup biasing the thread-engaging meansin a retracted position within the cylinder but upon sufficient pressureagainst the piston within the cylinder permitting the threadengagingmeans to travel a limited distance with the piston toward an extendedposition;

F. said thread-engaging means when in the retracted position holding thepiston out of contact with the pressure plate to minimize heat transferfrom the pressure plate to the piston. 10. An automatic brake ad usteras claimed in claim 9 wherein the threaded portion of the piston is in ahole located centrally in the piston and the thread-engaging means is athreaded stem having at least one split through the portion thereofwhich engages the threaded hole in the piston, the split permitting thestem to decrease in diameter a sufficient amount to permit the threadsof the stem to skip across the threads of the piston when the piston andstem are moved axially with respect to each other.

11. An automatic brake adjuster as claimed in claim 10 including a keymember attached to the piston and extending into one of the splits inthe stem in sliding relationship therewith to permit relative axialmovement of the stem and the piston but preventing relativecircumferential movement thereof.

1. An automatic brake adjuster for adjusting the distance of travel of abrake piston in a cylinder to compensate for wear of the brake frictionsurfaces comprising: A. a housing having a cylinder; B. a piston withinthe cylinder, said piston having a threaded portion; C. athread-engaging means for resiliently contacting at least part of thethreaded portion of the piston to permit relative axial movement of thecylinder and the thread-engaging means with respect to each otherwithout relative circumferential rotation of either of said parts; D.retaining means mounted within the cylinder for permitting limited axialmovement of the thread-engaging means with the piston upon applicationof the brake; and E. a sleeve threadably attached to the inner wall ofthe cylinder for holding the retaining means.
 2. An automatic brakeadjuster as claimed in claim 1 wherein the threaded portion of thepiston is in a hole located centrally in the piston and thethread-engaging means is a threaded stem having at least one splitthrough the portion thereof which engages the threaded hole in thepiston, the split permitting the stem to decrease in diameter asufficient amount to permit the threads of the stem to skip across thethreads of the piston when the piston and stem are moved axially withrespect to each other.
 3. An automatic brake adjuster as claimed inclaim 1 wherein the retaining means comprises a retaining cup mounted ina substantially fixed axial position within the cylinder and a springwithin the retaining cup biasing the thread-engaging means in aretracted position within the cylinder but upon sufficient pressureagainst the piston within the cylinder permitting the thread-engagingmeans to travel a limited distance with the piston toward an extendedposition.
 4. An automatic brake adjuster as claimed in claim 1 whereinthe piston is made of metal and has a nonmetallic insulator attached tothe axially outer end thereof.
 5. An automatic brake adjuster as claimedin claim 1 wherein the piston body is made from a nonmetallic insulatingmaterial and has a metal threaded insert located centrally thereof. 6.An automatic brake adjuster for adjusting the distance of travel of abrake piston in a cylinder to compensate for wear of the brake frictionsurfaces comprising: A. a housing having a cylinder; B. a piston withinthe cylinder, said piston having a threaded portion; C. athread-engaging means for resilientlY contacting at least part of thethreaded portion of the piston to permit relative axial movement of thecylinder and the thread-engaging means with respect to each otherwithout relative circumferential rotation of either of said parts; D.means mounted within the cylinder for permitting limited axial movementof the thread-engaging means with the piston upon application of thebrake; and E. a lock ring engaging an annular groove in the axiallyinner end of the cylinder for holding the retaining means.
 7. Anautomatic brake adjuster as claimed in claim 6 wherein the threadedportion of the piston is in a hole located centrally in the piston andthe thread-engaging means is a threaded stem having at least one splitthrough the portion thereof which engages the threaded hole in thepiston, the split permitting the stem to decrease in diameter asufficient amount to permit the threads of the stem to skip across thethreads of the piston when the piston and stem are moved axially withrespect to each other.
 8. An automatic brake adjuster as claimed inclaim 6 wherein the retaining means comprises a retaining cup mounted ina substantially fixed axial position within the cylinder and a springwithin the retaining cup biasing the thread-engaging means in aretracted position within the cylinder but upon sufficient pressureagainst the piston within the cylinder permitting the thread-engagingmeans to travel a limited distance with the piston toward an extendedposition.
 9. In a disc brake having a pressure plate for exertingpressure against the brake discs, an automatic brake adjuster foradjusting the distance of travel of a brake piston in a cylinder tocompensate for wear of the brake friction surfaces comprising: A. ahousing having a cylinder; B. a piston within the cylinder, said pistonhaving a threaded portion; C. a thread-engaging means for resilientlycontacting at least part of the threaded portion of the piston to permitrelative axial movement of the cylinder and the thread-engaging meanswith respect to each other without relative circumferential rotation ofeither of said parts; D. a retaining cup mounted in a substantiallyfixed axial position within the cylinder; and E. a spring within theretaining cup biasing the thread-engaging means in a retracted positionwithin the cylinder but upon sufficient pressure against the pistonwithin the cylinder permitting the thread-engaging means to travel alimited distance with the piston toward an extended position; F. saidthread-engaging means when in the retracted position holding the pistonout of contact with the pressure plate to minimize heat transfer fromthe pressure plate to the piston.
 10. An automatic brake adjuster asclaimed in claim 9 wherein the threaded portion of the piston is in ahole located centrally in the piston and the thread-engaging means is athreaded stem having at least one split through the portion thereofwhich engages the threaded hole in the piston, the split permitting thestem to decrease in diameter a sufficient amount to permit the threadsof the stem to skip across the threads of the piston when the piston andstem are moved axially with respect to each other.
 11. An automaticbrake adjuster as claimed in claim 10 including a key member attached tothe piston and extending into one of the splits in the stem in slidingrelationship therewith to permit relative axial movement of the stem andthe piston but preventing relative circumferential movement thereof.